Light-emitting Diode (LED) Driving Circuit

ABSTRACT

A light-emitting diode (LED) driving circuit for driving a plurality of first lightbars and a plurality of second lightbars is provided. The LED driving circuit includes a first current mirror, a second current mirror and a control circuit. The first current mirror, if enabled, balances currents of the first lightbars. The second current mirror, if enabled, balances currents of the second lightbars. During a first period, the control circuit disables the second current mirror and adjusts the duration of enabling the first current mirror according to a dimming signal. During a second period, the control circuit disables the first current mirror and adjusts the duration of enabling the second current mirror according to the dimming signal. Therefore, only the first lightbars or the second lightbars are driven in each period.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a division of U.S. application Ser. No.12/913,837 filed Oct. 28, 2010, which claims the priority benefit ofTaiwan application serial no. 98136410, filed Oct. 28, 2009, and Taiwanapplication serial no. 98138314, filed Nov. 11, 2009, the contents ofwhich are hereby incorporated by reference herein in their entireties.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a light-emitting diode (LED) drivingcircuit. More particularly, the present invention relates to an LEDdriving circuit for driving a plurality of lightbars each including aplurality of LEDs coupled in series.

2. Description of the Prior Art

FIG. 1 is a block diagram illustrating a conventional LED drivingcircuit. Referring to FIG. 1, an LED driving circuit 1 is adapted todriving a plurality of lightbars LB1-LBm, and each lightbar LBi includesa plurality of LEDs D1-Dn coupled in series, where m and n are positiveintegers, and i is an integer from 1 to m. The LED driving circuit 1includes a direct-current to direct-current (DC/DC) converter 11 and anLED controller 12. The DC/DC converter 11 such as a buck or boostconverter converts a DC input voltage Vin to a DC voltage V1 bsufficient to drive the lightbars LB1-LBm. Each lightbar LBi has a firstterminal coupled to the DC/DC converter 11 to receive the DC voltage V1b and a second terminal coupled to a corresponding channel terminal CHiof the LED controller 12. The LED controller 12 detects current of eachlightbar LBi and controls current of each lightbar LBi to become equalto a predetermined value by built-in constant current sources orvariable resistors; that is, the LED controller 12 balances currents ofthe lightbars LB1-LBm. The LED controller 12 further outputs a feedbacksignal from a feedback terminal FB to control the DC/DC converter 11 toadjust the DC voltage V1 b.

If too many lightbars LB1-LBm are employed, or if LEDs D1-Dn with highbrightness are employed, the total current of the lightbars LB1-LBm maycause the LED controller 12 to be destroyed. Accordingly, there is aneed for an LED driving circuit to employ external control manner shownin FIG. 2. Referring to FIG. 2, an LED driving circuit 2 includes aDC/DC converter 21, an LED controller 22, a plurality of transistorsM1-Mm and a plurality of resistors R1-Rm. The transistor Mi and theresistor Ri are coupled in series between the second terminal of acorresponding lightbar LBi and a ground. The LED controller 22 detectscurrent of each lightbar LBi from a corresponding current sensingterminal ISi and outputs a signal from a corresponding channel terminalCHi to control current of each lightbar LBi to become equal to apredetermined value. The LED controller 22 further detects a voltage atthe second terminal of each lightbar LBi from a voltage detectingterminal VDi to provide a short protection for the lightbars LB1-LBm.

The LED driving circuits 1 and 2 employ the LED controllers 12 and 22which are specific-purpose integrated circuits (ICs). However, acommercially available LED controller IC supports a fixed number oflightbars. It may be necessary to employ a plurality of LED controllerICs to drive the lightbars as the number of the lightbars increases. Thenumber of the transistors M1-Mm and the resistors R1-Rm employed in theLED driving circuit 2 will increase as the number of the lightbarsincreases. Therefore, as the number of the lightbars increases, theconventional LED driving circuits become more complex and expensive todesign and manufacture.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide an LEDdriving circuit employing a simple driving structure with reducedcomponents and without using a specific-purpose LED controller IC.

The present invention provides an LED driving circuit for driving aplurality of first lightbars and a plurality of second lightbars. Eachof the first lightbars and the second lightbars includes a plurality ofLEDs coupled in series. Each of the first lightbars and the secondlightbars has a first terminal coupled to receive a direct-current (DC)voltage and a second terminal. The LED driving circuit includes a firstcurrent mirror, a second current mirror and a control circuit. The firstcurrent mirror is coupled to the second terminals of the firstlightbars. The first current mirror balances currents of the firstlightbars when the first current mirror is enabled, and causes thecurrents of the first lightbars to become zero when the first currentmirror is disabled. The second current mirror is coupled to the secondterminals of the second lightbars. The second current mirror balancescurrents of the second lightbars when the second current mirror isenabled, and causes the currents of the second lightbars to become zerowhen the second current mirror is disabled. The control circuit iscoupled to the first current mirror and the second current mirror.During a first period, the control circuit disables the second currentmirror and adjusts the duration of enabling the first current mirroraccording to a dimming signal. During a second period, the controlcircuit disables the first current mirror and adjusts the duration ofenabling the second current minor according to the dimming signal. Thefirst period and the second period are repeated alternatively.

The control circuit includes a controller, a first switch, a firstcurrent detector, a second switch and a second current detector. Thecontroller outputs a first pulse-width modulation (PWM) signal and asecond PWM signal. A duty cycle of the first PWM signal is determined bythe dimming signal and the total current of the first lightbars, and aduty cycle of the second PWM signal is determined by the dimming signaland the total current of the second lightbars. The first switch has afirst terminal coupled to the first current mirror to receive the totalcurrent of the first lightbars, a second terminal and a control terminalcoupled to the controller to receive the first PWM signal. The firstcurrent detector has a first terminal coupled to the second terminal ofthe first switch and the controller and a second terminal coupled to aground. The first current detector detects the total current of thefirst lightbars. The second switch has a first terminal coupled to thesecond current mirror to receive the total current of the secondlightbars, a second terminal and a control terminal coupled to thecontroller to receive the second PWM signal. The second current detectorhas a first terminal coupled to the second terminal of the second switchand the controller and a second terminal coupled to the ground. Thesecond current detector detects the total current of the secondlightbars.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings are included to provide a furtherunderstanding of the invention, and are incorporated in and constitute apart of this specification. The drawings illustrate embodiments of theinvention and, together with the description, serve to explain theprinciples of the invention.

FIG. 1 is a block diagram illustrating a conventional LED drivingcircuit;

FIG. 2 is a block diagram illustrating another conventional LED drivingcircuit;

FIG. 3 is a block diagram illustrating an LED driving circuit accordingto a preferred embodiment of the present invention;

FIG. 4 is a schematic diagram illustrating the LED driving circuit shownin FIG. 3; and

FIG. 5 is a timing diagram illustrating time division control of the LEDdriving circuit shown in FIG. 4 under maximum brightness.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Reference will now be made in detail to the preferred embodiments of thepresent invention, examples of which are illustrated in the accompanyingdrawings. Wherever possible, the same reference numbers are used in thedrawings and the description to refer to the same or like parts.

FIG. 3 is a block diagram illustrating an LED driving circuit accordingto a preferred embodiment of the present invention. Referring to FIG. 3,an LED driving circuit 3 is adapted to driving a plurality of firstlightbars LB11-LB1 m and a plurality of second lightbars LB21-LB2 m,where m is a positive integer. Each of the first lightbars LB11-LB1 mand the second lightbars LB21-LB2 m includes a plurality of LEDs D1-Dncoupled in series, where n is a positive integer. Each of the firstlightbars LB11-LB1 m and the second lightbars LB21-LB2 m has a firstterminal coupled to receive a DC voltage V1 b and a second terminal. TheDC voltage V1 b, for example, is provided by the DC/DC converter 11shown in FIG. 1.

The LED driving circuit 3 includes a first current mirror 31, a secondcurrent mirror 32 and a control circuit 33. The first current mirror 31is coupled to the second terminals of the first lightbars LB11-LB1 m toreceive currents I11-I1 m of the first lightbars LB11-LB1 m. The firstcurrent mirror 31 balances the currents I11-I1 m of the first lightbarsLB11-LB1 m when the first current mirror 31 is enabled, and causes thecurrents I11-I1 m of the first lightbars LB11-LB1 m to become zero whenthe first current mirror 31 is disabled. The second current mirror 32 iscoupled to the second terminals of the second lightbars LB21-LB2 m toreceive currents I21-I2 m of the second lightbars LB21-LB2 m. The secondcurrent mirror 32 balances the currents I21-I2 m of the second lightbarsLB21-LB2 m when the second current mirror 32 is enabled, and causes thecurrents I21-I2 m of the second lightbars LB21-LB2 m to become zero whenthe second current mirror 32 is disabled. The control circuit 33 iscoupled to the first current mirror 31 and the second current mirror 32.During a first period, the control circuit 33 disables the secondcurrent mirror 32, and the control circuit 33 enables the first currentmirror 31 and adjusts the duration of enabling the first current mirror31 according to a dimming signal Vdim. During a second period, thecontrol circuit 33 disables the first current mirror 31, and the controlcircuit 33 enables the second current mirror 32 and adjusts the durationof enabling the second current mirror 32 according to the dimming signalVdim. The first period and the second period are repeated alternatively.

FIG. 4 is a schematic diagram illustrating the LED driving circuit shownin FIG. 3. Referring to FIG. 4, the first current mirror 31 includes aplurality of first transistors Q11-Q1 m. Each first transistor Q1 i hasa first terminal, a second terminal and a control terminal, where i isan integer from 1 to m. The first terminal of each first transistor Q1 iis coupled to the second terminal of a corresponding first lightbar LB1i. The second terminals of the first transistors Q11-Q1 m are coupled toone another and to the control circuit 33. The control terminals of thefirst transistors Q11-Q1 m are coupled to one another and to the firstterminal of one of the first transistors Q11-Q1 m (e.g. Q11). The secondcurrent mirror 32 includes a plurality of second transistors Q21-Q2 m.Each second transistor Q2 i has a first terminal, a second terminal anda control terminal. The first terminal of each second transistor Q2 i iscoupled to the second terminal of a corresponding second lightbar LB2 i.The second terminals of the second transistors Q21-Q2 m are coupled toone another and to the control circuit 33. The control terminals of thesecond transistors Q21-Q2 m are coupled to one another and to the firstterminal of one of the second transistors Q21-Q2 m (e.g. Q21). The firsttransistors Q11-Q1 m and the second transistors Q21-Q2 m are matched toone another.

The control circuit 33 includes a controller U1, a first switch M1, afirst current detector Rd1, a second switch M2 and a second currentdetector Rd2. The controller U1 outputs a first pulse-width modulation(PWM) signal Vpwm1 and a second PWM signal Vpwm2. A duty cycle of thefirst PWM signal Vpwm1 is determined by the dimming signal Vdim and thetotal current I31 of the first lightbars LB11-LB1 m, and a duty cycle ofthe second PWM signal Vpwm2 is determined by the dimming signal Vdim andthe total current I32 of the second lightbars LB21-LB2 m, in which thetotal current I31 is the sum of the currents I11-I1 m, and the totalcurrent I32 is the sum of the currents I21-I2 m. The first switch M1 hasa first terminal coupled to the second terminals of the firsttransistors Q11-Q1 m to receive the total current I31 of the firstlightbars LB11-LB1 m, a second terminal and a control terminal coupledto receive the first PWM signal Vpwm1. The first current detector Rd1has a first terminal coupled to the second terminal of the first switchM1 and the controller U1 and a second terminal coupled to a ground. Thefirst current detector Rd1 detects the total current I31 of the firstlightbars LB11-LB1 m. The second switch M2 has a first terminal coupledto the second terminals of the second transistors Q21-Q2 m to receivethe total current I32 of the second lightbars LB21-LB2 m, a secondterminal and a control terminal coupled to receive the second PWM signalVpwm2. The second current detector Rd2 has a first terminal coupled tothe second terminal of the second switch M2 and the controller U1 and asecond terminal coupled to the ground. The second current detector Rd2detects the total current I32 of the second lightbars LB21-LB2 m.

In this embodiment, the controller U1 is a general-purpose PWMcontroller IC such as TL494 or OZ9938. The controller U1 has a dimmingterminal DIM, output terminals G1 and G2 and a current sensing terminalIS. The controller U1 receives the dimming signal Vdim from the dimmingterminal DIM, obtains the detection result of the total current I31 ofthe first lightbars LB11-LB1 m and the total current I32 of the secondlightbars LB21-LB2 m from the current sensing terminal IS, and outputsthe first PWM signal Vpwm1 and the second PWM signal Vpwm2 from theoutput terminals G1 and G2. The first transistors Q11-Q1 m and thesecond transistors Q21-Q2 m are N-channel field-effect transistors(FETs). The first switch M1 and the second switch M2 are implemented byN-channel FETs. The first current detector Rd1 and the second currentdetector Rd2 are implemented by resistors. In an alternative embodiment,the first transistors Q11-Q1 m and the second transistors Q21-Q2 m areNPN bipolar junction transistors (BJTs), and the first switch M1 and thesecond switch M2 are implemented by NPN BJTs.

FIG. 5 is a timing diagram illustrating time division control of the LEDdriving circuit shown in FIG. 4 under maximum brightness. Referring toFIG. 5, a pulse signal Vpulse generated by an internal oscillator (notshown) of the controller U1 is served as an operating frequency of thecontroller U1. Time can be divided into a plurality of periods eachhaving a duration of T. Each period includes a pulse having a duration(or pulse width) of Td, where Td is much less than T. These pulses areadapted to avoiding overlap between the first PWM signal Vpwm1 and thesecond PWM signal Vpwm2 so that the first lightbars LB11-LB1 m areturned on when the second lightbars LB21-LB2 m are turned off, and thefirst lightbars LB11-LB1 m are turned off when the second lightbarsLB21-LB2 m are turned on. It is defined that odd periods are firstperiods T1 and even periods are second periods T2; hence, the firstperiod T1 and the second period T2 are repeated alternatively.

During the first period T1, the second PWM signal Vpwm2 remains at lowlevel and controls the second switch M2 to be open to disable the secondcurrent mirror 32, then the disabled second current mirror 32 controlsthe second lightbars LB21-LB2 m to be turned off and causes the currentsI21-I2 m of the second lightbars LB21-LB2 m to become zero (here, takingthe total current I32, the sum of the currents I21-I2m, as an exampleshown in FIG. 5). The first PWM signal Vpwm1 is at high level except inthe interval from j×2T to (j×2T+Td), where j is a non-negative integer.The first PWM signal Vpwm1 at high level of a duration Ton1 controls thefirst switch M1 to be closed to enable the first current mirror 31, thenthe enabled first current mirror 31 controls the first lightbarsLB11-LB1 m to be turned on and balances the currents I11-I1 m of thefirst lightbars LB 11-LB1 m (here, taking the total current I31, the sumof the currents I11-I1 m, as an example shown in FIG. 5). The first PWMsignal Vpwm1 at low level of a duration Td controls the first switch M1to be open to disable the first current mirror 31, then the disabledfirst current mirror 31 controls the first lightbars LB11-LB1 m to beturned off and causes the currents of the first lightbars LB11-LB1 m tobecome zero. The duty cycle of the first PWM signal Vpwm1, Ton1/2T, willbe adjusted by the controller U1 according to the dimming signal Vdimand the total current I31 of the first lightbars LB11-LB1 m; that is,the duration of enabling the first current mirror 31 or the duration ofthe first PWM signal Vpwm1 at high level, Ton1, will be adjusted.

During the second period T2, the first PWM signal Vpwm1 remains at lowlevel and controls the first switch M1 to be open to disable the firstcurrent mirror 31, then the disabled first current mirror 31 controlsthe first lightbars LB11-LB1 m to be turned off and causes the currentsI11-I1 m of the first lightbars LB11-LB1 m to become zero. The secondPWM signal Vpwm2 is at high level except in the interval from (2j+1)×Tto ((2j+1)×T+Td). The second PWM signal Vpwm2 at high level of aduration Ton2 controls the second switch M2 to be closed to enable thesecond current mirror 32, then the enabled second current mirror 32controls the second lightbars LB21-LB2 m to be turned on and balancesthe currents I21-I2 m of the second lightbars LB21-LB2m. The second PWMsignal Vpwm2 at low level of a duration Td controls the second switch M2to be open to disable the second current mirror 32, then the disabledsecond current mirror 32 controls the second lightbars LB21-LB2 m to beturned off and causes the currents I21-I2 m of the second lightbarsLB21-LB2 m to become zero. The duty cycle of the second PWM signalVpwm2, Ton2/2T, will be adjusted by the controller U1 according to thedimming signal Vdim and the total current I32 of the second lightbarsLB21-LB2 m; that is, the duration of enabling the second current mirror32 or the duration of the second PWM signal Vpwm2 at high level, Ton2,will be adjusted.

In this embodiment, the dimming signal Vdim is a DC signal whosemagnitude influences the duty cycles of the first PWM signal Vpwm1 andthe second PWM signal Vpwm2. In alternative embodiment, the dimmingsignal Vdim is a PWM signal whose duty cycle influences the duty cyclesof the first PWM signal Vpwm1 and the second PWM signal Vpwm2.

In summary, the LED driving circuit of the present invention divides alllightbars into the first lightbars and the second lightbars for timedivision control, and only the first lightbars or the second lightbarsare driven in each period. For example, only the first lightbars aredriven in the first period, and only the second lightbars are driven inthe second period. It reduces the amount of current provided by theDC/DC converter for driving the lightbars in each period; hence, theDC/DC converter can reduce voltage ripple therein and employ a filtercapacitor of smaller capacitance at its output to reduce its cost. Inaddition, the LED driving circuit of the present invention employs asimple driving structure with reduced components and without using aspecific-purpose LED controller IC to reduce its cost.

It will be apparent to those skilled in the art that variousmodifications and variations can be made to the structure of the presentinvention without departing from the scope or spirit of the invention.In view of the foregoing, it is intended that the present inventioncover modifications and variations of this invention provided they fallwithin the scope of the following claims and their equivalents.

I claim:
 1. A light-emitting diode (LED) driving circuit for driving aplurality of first lightbars and a plurality of second lightbars, eachof the first lightbars and the second lightbars comprising a pluralityof LEDs coupled in series, each of the first lightbars and the secondlightbars having a first terminal coupled to receive a direct-current(DC) voltage and a second terminal, the LED driving circuit comprising:a first current mirror coupled to the second terminals of the firstlightbars for balancing currents of the first lightbars when the firstcurrent mirror is enabled, and causing the currents of the firstlightbars to become zero when the first current mirror is disabled; asecond current mirror coupled to the second terminals of the secondlightbars for balancing currents of the second lightbars when the secondcurrent mirror is enabled, and causing the currents of the secondlightbars to become zero when the second current mirror is disabled; anda control circuit coupled to the first current mirror and the secondcurrent mirror for, during a first period, disabling the second currentmirror and adjusting the duration of enabling the first current mirroraccording to a dimming signal; and, during a second period, disablingthe first current mirror and adjusting the duration of enabling thesecond current mirror according to the dimming signal, the first periodand the second period being repeated alternatively; wherein the controlcircuit comprises: a controller for outputting a first pulse-widthmodulation (PWM) signal and a second PWM signal, a duty cycle of thefirst PWM signal being determined by the dimming signal and the totalcurrent of the first lightbars, a duty cycle of the second PWM signalbeing determined by the dimming signal and the total current of thesecond lightbars; a first switch having a first terminal coupled to thefirst current mirror to receive the total current of the firstlightbars, a second terminal and a control terminal coupled to thecontroller to receive the first PWM signal; a first current detectorhaving a first terminal coupled to the second terminal of the firstswitch and the controller and a second terminal coupled to a ground, thefirst current detector detecting the total current of the firstlightbars; a second switch having a first terminal coupled to the secondcurrent mirror to receive the total current of the second lightbars, asecond terminal and a control terminal coupled to the controller toreceive the second PWM signal; and a second current detector having afirst terminal coupled to the second terminal of the second switch andthe controller and a second terminal coupled to the ground, the secondcurrent detector detecting the total current of the second lightbars. 2.The LED driving circuit according to claim 1, wherein the first currentmirror comprises a plurality of first transistors, each first transistorhaving a first terminal, a second terminal and a control terminal, thefirst terminal of each first transistor being coupled to the secondterminal of a corresponding first lightbar, the second terminals of thefirst transistors being coupled to one another and to the controlcircuit, the control terminals of the first transistors being coupled toone another and to the first terminal of one of the first transistors;and wherein the second current mirror comprises a plurality of secondtransistors, each second transistor having a first terminal, a secondterminal and a control terminal, the first terminal of each secondtransistor being coupled to the second terminal of a correspondingsecond lightbar, the second terminals of the second transistors beingcoupled to one another and to the control circuit, the control terminalsof the second transistors being coupled to one another and to the firstterminal of one of the second transistors, the first transistors and thesecond transistors being matched to one another.
 3. The LED drivingcircuit according to claim 2, wherein the first transistors and thesecond transistors comprise NPN bipolar junction transistors (BJTs). 4.The LED driving circuit according to claim 2, wherein the firsttransistors and the second transistors comprise N-channel field-effecttransistors (FETs).
 5. The LED driving circuit according to claim 1,wherein the controller is a general-purpose PWM controller integratedcircuit.
 6. The LED driving circuit according to claim 1, wherein thefirst switch and the second switch are implemented by N-channel PETs. 7.The LED driving circuit according to claim 1, wherein the first currentdetector and the second current detector are implemented by resistors.